This invention relates to a method of manufacturing a semiconductor device, and in particular, a method of forming an electrode of a compound semiconductor device, such as, a Schottky junction field effect transistor (MESFET metal-semiconductor field effect transistor) and a hetero junction field effect transistor (HJFET).
Recently, devices which utilize high frequency of 0.5 GHz or more, such as, a portable telephone, a portable information terminal and a satellite communication I broadcast receiver have been widely used. In this event, a compound semiconductor device which is advantageous in performance, such as, electric power efficiency is used as the above device because a small size and lower power consumption are required in such devices.
Under such a circumstance, it is necessary that characteristics of the devices are uniform to each other to achieve designed performance in circuit devices which are mass-produced. The production of this device can not comply with manual adjustment of the characteristic in the course of a manufacturing step. Therefore, it is required to statistically control the step by the use of a processing apparatus having high accuracy, and namely to improve processing accuracy of each step. In addition, it is extremely important to apply a process which is capable of suppressing variation of the characteristic.
In the general method of manufacturing the MESFET, a gate electrode is formed by the use of the dry etching, such as, the reactive ion etching (RIE). Specifically, a semiconductor layer is exposed by forming a gate opening in an insulating film by the use of the RIE. In this event, when the semiconductor layer is exposed on the condition that anisotropy is strong, the channel crystal layer is damaged due to ion impact. In consequence, carriers on the surface thereof are unevenly reduced. In this case, it is difficult to recover the damage by the heat treatment. Further, the manufacturing yield is degraded because the device characteristic of the FET becomes uneven.
To solve this problem, a variety of suggestions have been made about the anisotropy dry-etching method which utilizes microwave with low damage, such as, the electron cyclotron resonance (ECR). By the using the ECR method, the etching can be carried out with low damage.
Meanwhile, the epitaxial apparatus for the mass-production has been developed with advancement of the compound semiconductor device. In particular, it is possible to produce a thin-film having high impurity concentration by the use of the metal organic chemical vapor deposition (MOCVD) with high accuracy and reproducibility. In this condition, a fine gate electrode is arranged on the conductive semiconductor layer of the thin-film having the high impurity concentration to produce the FET having high transconductance gm and cut-off frequency f.sub.T with high reproducibility.
Therefore, the ECR dry etching has been attempted to be utilized by using the epitaxial semiconductor substrate which is fabricated by the use of the MOCVD. Namely, when the insulating film is etched or opened, the RIE method is first carried out until the insulating film of 100 nm is left so as to not damage the semiconductor layer. Thereafter, the ECR method is performed to expose the semiconductor layer.
However, reacted product is generated on the surface of the semiconductor layer (namely, channel layer) after the etching. Where the reacted product is left on the Schottky electrode forming region, a forward current does not flow even when the forward bias is applied by forming the Schottky electrode. Therefore, an excellent Schottky barrier is not formed in this condition. Consequently, the gate function can be obtained.
On the other hand, suggestion has been made about another method of forming the gate electrode. In this method, after the insulating film is left with the preselected region, the wet etching is carried out to expose the semiconductor layer. In this case, the two kinds of insulating films are deposited to prevent the opening from unevenly widening by the isotropic wet etching.
However, in this method, the side etching takes place in the insulating film by the use of the wet etching. Consequently, vaporized electrode metal is cut out with the step-shape in the side-etching portion.
Further, suggestion has been made about the other method of forming the gate electrode. In this method, a sloped side surface is used in order to improve the connection of the vaporized metal in the gate-opening portion. Thereby, a reduced gate opening which has the sloped side surface on an upper portion of the opening can be formed on the condition that the sloped curve surface is kept.
However, the side etching brings about in the bottom portion because the isotropic etching is carried out for the insulating film. Consequently, the vaporized metal does not almost enter into the opening. As a result, the metal of the side surface becomes thin and thereby, the series resistance becomes large.
As mentioned before, when the semiconductor layer (crystal surface of compound semiconductor) is exposed by the dry etching, the reacted product is formed by the crystal components and the carbon. Consequently, the excellent Schottky barrier can not be formed.
Further, when the electrode is formed in the fine opening, it is difficult that the vaporized electrode metal entirely or completely attaches on the side surface. In consequence, the electrode inevitably becomes thin. As a result, the series resistance is increased and the disconnection of the electrode often takes place.